Casting is a process of making a cast, and proceeds in the sequence of preparation of a cast design, selection of a casting method, preparation of a mold, melting and pouring of a material, and finishing.
Non-ferrous metal casting methods include a gravity die casting method and a low pressure die casting method. The gravity die casting method and the low pressure die casting method have an advantage in that they enable manufacture of various shapes and sizes of products, but disadvantageously require high mold and molten material temperatures and an excessively long solidification time, thus causing inclusion of air bubbles into a product and generating cracks due to volume reduction during solidification.
As described above, the gravity die casting method and the low pressure die casting method entail deterioration in the density of a completed product and in mechanical properties and therefore, have failed to satisfy quality requirements of related industrial fields. In particular, the gravity die casting method and the low pressure die casting method could not comply with the recent tendency of automotive industries to manufacture relatively large and complex automotive components.
Despite the above described problems, currently, domestic and foreign automotive component manufacturers are manufacturing a great quantity of non-ferrous metal products, such as aluminum wheels, by use of the gravity die casting method and the low pressure die casting method. Use of these die casting methods consequently increases product defect rates and deteriorates mechanical properties of products, such as, e.g., cast structure, rigidity, elongation. For this reason, conventional gravity die casting method and low pressure die casting methods have been applied to manufacture of a limited range of products, such as small aluminum wheels (up to a maximum of 18 inches). In addition, delay of a product casting cycle time may occur since it is necessary to keep a mold temperature in the range of about 380° C. to 400° C. due to characteristics of the die casting methods and consequently, the gravity die casting method and the low pressure casting methods are unsuitable for mass production.
To solve the above described problems and to realize a low product weight, hot forging methods have been developed.
In a hot forging method to form an aluminum wheel, an aluminum billet is subjected to a cutting process, a heating process and a forming/pressurizing process using a hydraulic press in sequence. Due to adoption of direct casting of a solid material, the hot forging method may problematically require that casting processes be performed repeatedly, e.g., three or four times when it is attempted to form a complex product.
Furthermore, since it is necessary to repeatedly perform a thermal treatment several times in order to prevent work hardening, the hot forging method suffers from troublesome processes, high manufacturing costs, and increased facility costs for mass production, resulting in an increased product price. In conclusion, the hot forging method has difficulty in practical application for passenger motor vehicles and thus, has been used to manufacture wheels of commercial vehicles having relatively simple designs.